A trek is physically demanding because of its length and the almost unbelievable changes in elevation. During the 300-km trek from Jiri to Everest base camp and return, for example, the trail gains and loses more than 9000 meters of elevation during many steep ascents and descents. On most treks, the daily gain is less than 800 meters in about 15 km, though ascents of as much as 1200 meters are possible on some days. You can always take plenty of time during the day to cover this distance, so the physical exertion, though quite strenuous at times, is not sustained. You also can stop frequently and take plenty of time for rest.
Probably the only physical problem that may make a trek impossible is a history of knee problems on descents. In Nepal the descents are long, steep and unrelenting. There is hardly a level stretch of trail in the entire country. If you are an experienced walker and often hike 15 km a day with a pack, a trek should prove no difficulty. You will be pleasantly surprised at how easy the hiking can be if you only carry a light backpack and do not have to worry about meal preparation.
Previous experience in hiking and living outdoors is, however, helpful as you make plans for your trek. The first night of a month-long trip is too late to discover that you do not like to sleep in a sleeping bag. Mountaineering experience is not necessary, but you must enjoy walking.